Optical fiber lasers have received increasing attention in recent years, particularly as the source for pumping optical fiber amplifiers. Such lasers typically include a core which is doped by an impurity such as Nd ions so that pumping the core through the cladding with a source such as a laser array or broad strip laser causes an absorption by the dopants which results in emission of the energy in another wavelength spectrum allowing lasing to take place in the core when appropriate reflecting means are used. Most popular designs use a core which is doped with Nd ions or Yb ions and is pumped with light having a range of wavelengths of 800-870 nm or 900-960 nm, respectively. Nd -doped lasers have the advantage of low lasing thresholds (typically about 100 mW), but typically exhibit optical conversion efficiencies of only 35-45 percent. Yb-doped lasers typically have conversion efficiencies of 60-75 percent, but have high lasing thresholds, e.g., as high as several hundred milliwatts.
The core could be co-doped with Nd and Yb ions so that one dopant is pumped and the energy is transferred to the other dopant in order to increase the choice of pump wavelength. (See, e.g., U.S. Pat. No. 4,815,079 issued to Snitzer et al, and U.S. Pat. No. 5,225,925 issued to Grubb et al.) It has also been suggested that a plurality of laser sections could be formed in a single fiber with the sections separated by gratings formed in the fiber. (See, e.g., U.S. Pat. No. 5,473,622 issued to Grubb.) It has been further suggested that more than one fiber laser could share a common grating. (See, e.g., of Grubb et al., U.S. patent provisional application Ser. No. 60/026,906, filed Sep. 26, 1996, and DiGiovanni et al., U.S. patent provisional application Ser. No. 60/026,907, filed Sep. 26, 1996.)
In spite of such suggestions, there still exists the need for high efficiency and low lasing threshold in fiber lasers.